* Add bindings for serialization.
* Change `xgb.save.raw' into full serialization instead of simple model.
* Add `xgb.load.raw' for unserialization.
* Run devtools.
* [R] add native routines registration
* c_api.h needs to include <cstdint> since it uses fixed width integer types
* [R] use registered native routines from R code
* [R] bump version; add info on native routine registration to the contributors guide
* make lint happy
